The proliferation of portable electronic devices such as cell phones and laptop computers has lead to an increased demand for high capacity, long endurance light weight batteries. Because of this alkali metal batteries, especially lithium ion batteries, have become a useful and desirable energy source. Lithium metal, sodium metal, and magnesium metal batteries are well known and desirable energy sources.
By way of example and generally speaking, lithium batteries are prepared from one or more lithium electrochemical cells containing electrochemically active (electroactive) materials. Such cells typically include, at least, a negative electrode, a positive electrode, and an electrolyte for facilitating movement of ionic charge carriers between the negative and positive electrode. As the cell is charged, lithium ions are transferred from the positive electrode to the electrolyte and, concurrently from the electrolyte to the negative electrode. During discharge, the lithium ions are transferred from the negative electrode to the electrolyte and, concurrently from the electrolyte back to the positive electrode. Thus with each charge/discharge cycle the lithium ions are transported between the electrodes. Such rechargeable batteries are called rechargeable lithium ion batteries or rocking chair batteries.
The electrodes of such batteries generally include an electrochemically active material having a crystal lattice structure or framework from which ions, such as lithium ions, can be extracted and subsequently reinserted and/or permit ions such as lithium ions to be inserted or intercalated and subsequently extracted. Recently, a class of transition metal phosphates and mixed metal phosphates have been developed, which have such a crystal lattice structure. These transition metal phosphates are insertion based compounds like their oxide based counterparts. The transition metal phosphates and mixed metal phosphates allow great flexibility in the design of lithium ion batteries.
Examples of such transition metal phosphates and mixed metal phosphates include olivine LiMPO4 compounds, wherein M is Fe, Mn or Co. Examples of the mixed metal phosphates include olivine compounds of the formula LiMI1-xMIIxPO4, wherein MI is selected from the group consisting of Fe, Co, and Mn or mixtures thereof and MII is selected from the group consisting of Mg, Ca, Zn, Ni, Co, Cu, Al, B, Cr, and Nb and x is from about 0.01 to about 0.15.
Although these compounds find use as electrochemically active materials useful for producing electrodes these materials are not always economical to produce. They may afford insufficient voltage, have insufficient charge capacity or exhibit low ionic conductivity. The present invention provides an economical method for producing metal phosphates and mixed metal phosphates with electrochemical properties which make them useful for producing electrodes and in particular cathodes. The metal phosphates and mixed metal phosphates produced in the present invention contain less residual carbon and therefore can produce materials of higher energy density then materials previously known.